This service repair manual covers MerCruiser PCM 555 Diagnostic sterndrive and inboard models, including the 496 MAG HO and 496 MAG sterndrives, and the 8.1S HO and 8.1S Horizon inboard engines, all with serial numbers starting from 0M000000. These V8 gasoline engines, with 8.1-liter (496 cubic inch) displacement, offer approximately 425 horsepower for HO models and 370 horsepower for standard models, designed for robust marine applications. The manual details the engineering and reliability inherent in these powerful marine propulsion systems. This comprehensive factory service manual provides essential information for the overhaul, repair, inspection, adjustment, and maintenance of your MerCruiser engine. It includes detailed sections on general information, troubleshooting, and wiring diagrams, along with guidance on lubrication and cooling system requirements. This manual is an indispensable resource for owners, mechanics, and technicians to ensure the optimal performance and longevity of their MerCruiser sterndrive and inboard engines.